1. Field of the Invention
The present invention relates to a pattern drawing method using charged particle beams for forming a fine pattern on a semiconductor substrate, and relates, more particularly, to a pattern drawing method using charged particle beams for forming a fine pattern by dividing the pattern to be drawn into a central section and an outer edge section surrounding the central section and irradiating charged particle beams to these sections separately, and an apparatus for the pattern drawing method.
2. Description of the Related Art
Along with the advancement of LSIs, there have been progressed rapidly methods for providing fine patterns to be used for semiconductor devices. In order to manufacture a semiconductor device having an inter-pattern distance of 0.25 .mu.m or less which is required from now on, a pattern drawing method using charged particle beams is an effective method. FIG. 1 is a schematic diagram for showing a pattern to be drawn. Two rectangular pattern scheduled areas 21 having 20 .mu.m as a short side length and 100 .mu.m as a long side length are provided with their long sides set parallel with each other and with a 0.2 .mu.m distance between these two areas. A pattern is scheduled to be drawn in these pattern scheduled areas 21 by using electron beams. In other words, there is provided a space area 33 in which no pattern is scheduled to be formed between the pattern scheduled areas 21.
This, however, has a problem in that when electron beams of the same dose is applied uniformly to these pattern scheduled areas 21, the space area 33 is buried by the pattern. This is a phenomenon which occurs due to a dispersion of electron beams within a resist and on the surface of a substrate, the so-called proximity effect. This problem then becomes more serious as the pattern to be drawn becomes more minute.
Under the above circumstances, there has been proposed a method for dividing each of the pattern scheduled areas 21 into an outer edge section for forming an outer edge of this area and a central section surrounded by the outer edge section, and for setting the dose in the outer edge section to be higher than that of the central section. FIG. 2 is a schematic diagram for showing a pattern to be drawn divided into the outer edge section and the central section. As shown in FIG. 2, each of the two pattern scheduled areas 21 shown in FIG. 1 is divided into an outer edge section 23 having a width of 0.2 .mu.m provided at the outer periphery of the area and a central section 22 surrounded by the outer edge section 23. The beam exposure level in the outer edge section 23 is set higher than that in the central section 22, and electron beams are applied to these areas individually to form a pattern in each area.
This method, however, also has a problem in that the width of the space area 33 becomes larger at a position of the space area 33 far from its center portion in the longitudinal direction. FIG. 3 shows variations of the width of the space area 33 when a pattern is formed in the manner as described above by coating a negative resist of a thickness 0.5 .mu.m on a silicon substrate. FIG. 3 is a graphical representation for showing a relationship between a distance from the end portion to the center portion of the space area in the longitudinal direction as expressed in the horizontal axis and the width of the space area as expressed in the vertical axis, based on the prior-art example. As shown in FIG. 3, under an exposure condition that the width of the space area at the center portion is a design size of 0.2 .mu.m, the width of the space area becomes larger from a position of about 10 .mu.m distance from the end portion toward the end portion of the space area. Then, the width of the space area 33 at its end portion is larger than that at the center portion by 0.3 .mu.m or more. When such a size variation occurs, there arises a significantly serious problem that the properties of the semiconductor device change.
There has been proposed a pattern drawing apparatus for preventing an excess exposure in the contacting portion when two pattern scheduled areas are contacted with each other (Japanese Unexamined Patent Publication (Kokai) No. Hei 5-217869). According to this prior-art technique, when two pattern scheduled areas are contacted with each other, the lengths of the sides of these areas forming the contacting portion are compared, and the exposure of electron beams to the outer edge section corresponding to the side having a shorter length is omitted. With this arrangement, a reduction in the drawing accuracy due to excess dosage can be prevented as compared with the case where electron beams are applied to all the outer edge sections of each area.
There has also been proposed a pattern drawing method for adjusting the width of the outer edge section according to the pattern scheduled area (Japanese Unexamined Patent Publication (Kokai) No. Sho 59-167018). In this prior-art technique, a leakage of cumulated energy of electron beams to the outside of the pattern scheduled area is prevented by adjusting the width of the outer edge section according to the size of the pattern scheduled area. With this arrangement, the accuracy of drawing a pattern can be improved.
However, the phenomenon of the expansion of the width of the space area can not be prevented sufficiently even by these methods.